Ligand hemilabile

Various bifunctional potentially hemilabile ligands (72) bearing phosphorus groups were prepared and their coordination to rhodium was studied. Their effect on the hydroformylation of styrene was assessed.237... 

Figure 1 Chelating and hemilabile ligands used in palladium-catalyzed amination of aryl halides.

Mechanistic details of this reaction are scarce, but Aratani (14) mentions that the catalyst needs to be activated by heating in the presence of the diazo compound at 75-80°C until nitrogen evolution is observed and the color of the complex changes from green to brown. Reduction of the cupric precatalyst with a substituted hydrazine results in a yellow cuprous complex capable of inducing an instantaneous decomposition of diazoacetate at ambient temperature. Aratani proposes that the active catalyst is tetrahedral Cu(I), 26 in Scheme 2. Reaction with the diazoester from the less hindered face forms the Cu carbenoid having one hemilabile ligand (al-... 

The coordination of styrene with the pyrazole detached resulted in the formation of a stable n-complex (vide infra). Therefore, throughout the catalytic cycle the pyrazole plays the role of a hemilabile ligand [55], This is an important result because experimental studies [11] suggest that olefin coordination is slow and reversible. Moreover, the tuning of the steric bulk of the pyrazole substituents could be used to enhance the activity of the... 

A. L. Terthienyl and Poly-terthienyl Ligands as Redox-Switchable Hemilabile Ligands for Oxidation-State-Dependent Molecular Uptake and Release. J. Am. Chem. Soc. 2001, 123, 2503-2516. 

The problem raised by the deactivation of vacant site could be solved by the design of a protective device such as a redox-switchabk hemilabile ligand, RHL [62]. Ideally, the protective device should (1) be activated by an electronic (redox) or chemical (pH) instruction, (2) open the binding site only in the presence of the substrate, (3) function reversibly so that the site is protected back when the bound substrate is lost (due to reversibility of the binding step or due to decoordination during subsequent steps). An example of an electrochemically... 

Pitter S, Dinjus E (1997) Phosphinoalkyl nitriles as hemilabile ligands new aspects in the homogeneous catalytic coupling of C02 and 1,3-butadiene. J Mol Catal A Chem 125 39-45... 

The rationale behind the introduction of the ortto-methoxy group is its ability to behave as a hemilabile ligand, which is likely to stabilise the paUadium(O) catalyst species - after reductive elimination of the product in the catalytic cycle - by intermittently occupying the otherwise empty coordination sites preventing formation of palladium black. 

Phosphorus Heterocycles, and Phosphorus-based Hemilabile Ligands... 

In complexes of this type, there is an effective rotation of the 1,3-diene. This should be distinguished from the apparent rotation often found for 1,5-dienes, such as COD, in which ligands are interchanged relative to double bonds by M-P or M-N bond rupture with hemilabile ligands (see Section 6). 

There has been an increased interest in mixed-donor and hemilabile ligands owing to their successful use in catalysis owing to their electronic asymmetry or ability to provide access to an unsaturated site via dissociation of one of the termini. Typical ligands used for catalysis that make use of their hemilability involve a phosphine and an oxygen donor, such as an ether. A weak donor, such as an ether (if it binds at all), is relatively easily displaced, and rapture of the M-0 bond and retention of the M-P bond is expected to provide an ideal hemilabile system. With P,N- ligands, either... 

Table 2 Effect of counterions on the hydrovinylation of styrene using hemilabile ligand...

Interestingly, the monodentate phosphine-modified Ni and Pd catalysts respond differently to variation of the counterion the best results for the Ni catalysts are obtained in the presence of weakly coordinating counterions such as OTf whereas the analogous Pd catalysts require the presence of a noncoordinating counterion such as BAr4 or SbFe". The latter are also the preferred anions for Ni-catalysts modified by 1 or the hemilabile ligand 27 [75-77, 80]. 

---